From: Brett Buck <brett.buck@lmco.com>
Newsgroups: sci.space.policy,sci.astro
Subject: Cat beds heaters and monoprop performance characteristics WAS:( MPL
Date: Fri, 31 Mar 2000 12:48:10 -0800
Robert Clark wrote:
> So MCO and quite likely also MGS had catbed heaters for their
> hydrazine thrusters. It is therefore quite mystifying why it was felt
> that heaters were not required for the catbeds of the hydrazine
> thrusters on the MPL lander.
Blowing out the beds aside, the main reason for cat bed heaters on
monoprop thrusters is to avoid poor performance at startup. The first
pulse on a cold cat bed are weak and have large delays from the valve to
centroid of the thrust. This is a very serious concern of systems where
the thruster is used for infrequent attitude control pulsing, as the bed
could potentially cool down between pulses, and result in very poor or
unstable performance. Even one short pulse typically warms up the bed
enough that subsequent pulses are OK, as long as they are frequent
enough to avoid it cooling down in between.
The MPL descent engines used the monoprop thrusters in a mode that
they pulse very frequently (10 hz) in duty cycles ranging from 25% up. I
can easily believe that no heater would be necessary from a performance
standpoint, as the first pulse would warm up the bed, and the thousands
afterwards would be acceptable as a result. The very first pulse being
ratty would not be a significant issue, hence no cat bed heaters.
The damage issue is of course a concern, but the analysis and
decisions made to mitigate it are not dissimilar from many, many other
similar decision of a that nature that arise in this type of project. I
am surprised that cold cat beds caused permanent damage, but most of my
experience has been with smaller engines. A 60 lb monoprop is one big motor!
Brett